What about an FB25 makes it a non-drop-in replacement for the EJ25 NA motor?

The 2010 Legacy/Outback debuted an engine-cradle arrangment. The engine cradle was meant to offer modularity as to which Subaru engine fits in that chassis.

There is pretty much a ZERO percent chance that the '10 Legacy Engine cradle design was designed to preclude the FB engine fitment, as the FB engine was on the docket for 2011, in the Forester, and has been planned to entirely replace the EJ engine line.

The FB25 Forester uses the age-old Subaru 5-speed manual, and 4-speed auto... it bolts to the same transmissions. It bolts to the same engine mounts, and is likely designed for easy use with the engine cradle, as mentioned.

The engine cradle was more for the transmission. The CVT is considerably larger than the other transmissions. It also allowed better engine mount placement for less vibration.